Add cross-compilation support

[semlanik]

During cross-compilation generator should always be compiled for host system, but not for target

[rapgenic]

Could this be related to cmake variable QT_PROTOBUF_EXECUTABLE?

Because I tried using it for cross compilation but i have not been able to make it work and cmake gives me the following message:

/home/giulio/....../qtprotobuftest/build_arm/qtprotobuf/src/generator/qtprotobufgen: program not found or is not executable
Please specify a program using absolute path or make sure the program is available in your PATH system variable

Which seems to suggest that cmake ignores this variable.

Note that qtprotobuf is included via CMakeLists.txt into my project and that I tried passing the variable both as parameter to cmake (-DQT_PROTOBUF_EXECUTABLE=.....) and with set inside the lists file

[semlanik]

@rapgenic, unfortunately no. The problem is that qtprotobufgen compiled for ARM and couldn't be executed in x84/amd64 platform. I didn't find any CMake way to make compilation for host and native platforms in single CMake stack. I'm working on this task right now, and the only way I see is split generator and libraries deeper and make 2 steps crosscompilation. If you have an idea how it could be solved I'm open for suggestions :)

[rapgenic]

The problem is that qtprotobufgen compiled for ARM and couldn't be executed in x84/amd64 platform.

That is what i had noticed too, and I thought that I could compile once for x86 and then use QT_PROTOBUF_EXECUTABLE to have cmake use the x86 compiled generator instead of the ARM built one, but I could not get it to work. (this would be the best solution for me, I think, because I am going to have in my filesystem the right version of the generator already installed)

If you have an idea how it could be solved I'm open for suggestions :)

I think i have read somewhere that cmake can only use one toolchain... :(
Anyway i found this link on SO https://stackoverflow.com/questions/36173840/how-to-instruct-cmake-to-use-the-build-architecture-compiler which seems to suggest that running another cmake instance inside the main one might be the solution.

[semlanik]

As far as I could see, they use exact same approach that I have. Run cmake twice and use x86 version in compilation for ARM. Looks like it's the only right way to achieve cross-compilation.